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Effect of scanning strategy on mechanical properties of selective laser melted Inconel 718
Materials Science & Engineering A 753 (2019) 42–48
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of scanning strategy on mechanical properties of selective laser melted Inconel 718
T
H.Y. Wana,b, Z.J. Zhouc, C.P. Lic, G.F. Chenc, G.P. Zhanga,∗ a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016, China c Materials & Manufacturing Qualification Group, Corporate Technology, Siemens Ltd., China, Beijing, 100102, China b
ARTICLE INFO
ABSTRACT
Keywords: Selective laser melting Inconel 718 Scanning strategy Grain size Mechanical properties
The effect of scanning strategy, i.e. bidirectional scanning without (SS-X) and with a 90°-rotation (SS-XY) for every layer, on mechanical properties of Inconel 718 fabricated by selective laser melting (SLM) was investigated. The results show that tensile strength and fatigue strength of SS-X specimens are superior to that of the SS-XY ones. Such excellent mechanical properties of the SS-X specimens at room temperature were found to mainly result from the processing-induced fine grain structures compared with void size, crystalline orientation or dendrite structure.
1. Introduction
2. Experimental
Selective laser melting (SLM), is one of the promising additive manufacturing (AM) technologies, which is capable of fabricating nearnet shape metallic components with geometrically complex structures [1,2]. Despite many advantages, SLM process still faces a lot of challenges that need to be addressed prior to widespread industrial application. Two main challenges are microstructure heterogeneities and randomly dispersed defects, which result in the uncertainty and degradation in mechanical properties [3,4]. To obtain the final components with high density and optimal mechanical properties, much effort has been devoted to optimize the processing parameters, such as laser energy density [5,6], building orientation [7], and scanning strategy [8,9]. Scanning strategy is an important variable that significantly affects the thermal history during the SLM process and further influences the specimen density [10], residual stress [11] and evolution of the microstructures [8], ultimately changes the mechanical properties [12]. Most previous researchers focused on controlling the solidification microstructure and crystalline orientation by tailoring the scanning strategy [9,13–15], while few studies are available on the effect of the scanning strategy on mechanical properties, especially fatigue properties [8,9,12]. In this paper, two types of scanning strategies of SLM were adopted to produce the Inconel 718 specimens. The effects of void size distribution, phase composition and dendrite structure, crystalline orientation, as well as grain structure induced by scanning strategy on tensile and fatigue properties were comprehensively discussed.
2.1. Specimen preparation
∗
Specimens with the dimension of 10 × 10 × 10 mm3 were fabricated by an SLM apparatus (EOSINT M280 400W, EOS, Germany) equipped with an Yb fiber laser source under an argon atmosphere with an oxygen level of 0.1%. As-received Inconel 718 spherical metal powder with a size ranging from 15 μm to 53 μm was produced by Ar gas-atomization and the chemical composition was presented in our previous work [16]. The SLM process parameters, such as laser power, scanning speed, hatch distance and layer thickness were shown in Table 1. These process parameters were optimized according to the following criteria. Firstly, no obvious cracking and delamination can be found. Secondly, no obvious unmelt powder can be found on the surface of build parts. Thirdly, the profile depth, Pt (the sum of the largest profile peak height and the largest profile valley depth within the evaluation length) used to evaluate the surface roughness should be lower than 200 μm. Finally, the relative density should be larger than 99.5%. In order to investigate the effect of scanning strategy on mechanical properties of SLM-fabricated Inconel 718, two types of scanning strategies, bidirectional scanning without and with a 90° rotation between the successive layers, were adopted, which were named as SS-X and SS-XY, respectively. 2.2. Material characterization The size and distribution of voids were detected by the 3D-high
Corresponding author. E-mail address: [email protected] (G.P. Zhang).
https://doi.org/10.1016/j.msea.2019.03.007 Received 11 June 2018; Received in revised form 16 December 2018; Accepted 1 March 2019 Available online 07 March 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 753 (2019) 42–48
H.Y. Wan, et al.
Table 1 Process parameters of the SLM-fabricated Inconel 718. Sample
Laser power (W)
Scanning speed (mm/s)
Hatch distance (mm)
Layer thickness (mm)
Platform temperature (°C)
E = P/vht (J/mm3)
Scanning strategy
SS X SS XY
285 285
960 960
0.1 0.1
0.04 0.04
80 80
74.2 74.2
S-parallel (0°) S-cross (90°)
resolution transmission X-ray tomography (XRT) technique. The pixel size of 2.1 μm was selected depending on the specimen volume. Electron backscatter diffraction (EBSD) was performed to analyze the grain structure and crystalline orientation, the threshold value for the grain size was a misorientation of 10°. Dendrite structures were characterized by a field-emission scanning electron microscope (FE-SEM, LEO Supra 35) and the second phases were determined by a fieldemission transmission electron microscope (FE-TEM, FEI Tecnai 20) with an energy dispersive X-ray spectroscopy (EDS) attachment. 2.3. Tensile and fatigue test Dog-bone shaped tensile specimens with gauge dimensions of 3 mm in length, 1 mm in width and 200 μm in thickness were prepared. Tensile tests were conducted at a strain rate of 5 × 10−4 s−1 along building direction (BD) of the SLM-fabricated Inconel 718 using an INSTRON 5848 universal test machine with a 2 kN load cell at room temperature. The tensile strain was measured by a non-contact laser extensometer (MTS LX 300). Fatigue test specimens have a hourglassshaped test section with dimensions of 1 mm in width and 200 μm in thickness. Fatigue tests were also performed along BD at room temperature in an INSTRON E 1000. A stress ratio and loading frequency is 0.1 and 30 Hz, respectively. Both tensile and fatigue specimens were extracted from the inside of build cubes and mechanically polished followed by electropolished for 1 min at 12 V in a 10% perchloric acid solution to avoid the effect of surface defects. The side surface and fracture surface of failure specimens were characterized by the FE-SEM. 3. Results 3.1. Void distribution and microstructure Fig. 1(a) and (b) show 3D-XRT images on the SS-X and SS-XY specimens, respectively, indicating that a large number of micron-sized spherical voids uniformly distributed in both types of specimens. The relative density of the SS-X and SS-XY specimens is calculated as 99.85% and 99.81%, respectively. Fig. 1(c) shows that the size and distribution of voids have no remarkable difference. The mean void size of the SS-X and the SS-XY specimens is 9.3 ± 5.6 μm and 9.1 ± 5.5 μm, respectively. Fig. 2(a) and (b) present the grain structure and crystallographic texture of the SLM-fabricated Inconel 718 from the YZ cross section, respectively. The grain structure of the SS-X and SS-XY specimens is distinctive and the mean intercept length perpendicular to the BD is calculated as 19.06 μm and 43.90 μm, respectively (see Fig. 2(a) and (b)). While the mean width of dendrite structure shown in Fig. 2(c) and (d) is almost same (∼500 nm). As shown in the insets of Fig. 2(a) and (b), crystallographic textures of the SS-X and SS-XY specimens were evidently different. A weak texture was developed when scanning strategy X was applied. It is because that the elongated direction of the cells largely deviated from the plane perpendicular to the scanning direction (X) due to the complex heat flux direction, which is caused by the combined effects of Marangoni flow and vapor recoil pressure [14]. On the contrary, scanning strategy XY gave rise to a strong cube texture, which is different from that by Ishimoto et al. in which strong < 001 > alignments along the X, Y and Z directions [13]. It is also unexpected to find that the macroscopic morphology on the XZ plane and YZ plane in scanning strategy XY are significantly different, as shown in Fig. 3(a) and (b). The alternative morphology of the laser tracks and semi-ellipse
Fig. 1. 3D-XRT images showing the distribution of the voids of the (a) SS-X and (b) SS-XY specimens, (c) void size distribution of both types of specimens.
shaped melt pools can hardly be found. Instead, the laser tracks can only be found on the XZ plane (Fig. 3(a)) and the semi-ellipse shaped melt pools can only be observed on the YZ plane (Fig. 3(b)). However, the semi-ellipse shaped melt pools are visible on the top deposited layers on the XZ plane (Fig. 3(c) and (d)), which is similar to that found on the YZ plane (Fig. 3(b)). Therefore, the differences (macroscopic morphology and texture) between the XZ plane and YZ plane in the scanning strategy XY in our present work are attributed to other factors. 43
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Fig. 2. Band contrast (BC) maps showing grain structure of the (a) SS-X and (b) SS-XY specimens from the YZ cross section, insets in (a) and (b): pole figures of the SSX and SS-XY specimens, respectively, high-magnification SEM backscatter electron images of the dendrite structure of the (c) SS-X and (d) SS-XY specimens. Fig. 3. Cross-sectional pictures taken in the (a) XZ plane and (b) YZ plane of the SS-XY specimen shown the laser track morphology and semi-ellipse shaped melt pool morphology, respectively, (c) image of the cross-sections of the top layers taken in the XZ plane of the SS-XY specimen, (d) magnified images of the area enclosed in red rectangle in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
It should be noted that, besides the scanning strategy, the amount of partial remelting of the previously deposited layers and/or neighboring tracks (depending on the material system and specific processing conditions, such as scanning speed and layer thickness) would influence on the formation of macroscopic morphology and texture [17]. If the next deposited layer does not fully remelt the previously deposited layer, the alternative morphology of laser tracks and semi-ellipse melt pools can be observed on both XZ and YZ planes. And the mechanism for the texture formation in the scanning strategy XY will follow Ishimoto's opinion. By contrast, if the next deposited layer almost fully remelts the previously deposited layer, the macroscopic morphology on the XZ and YZ planes will be different. In other words, the laser tracks are mainly observed on the XZ plane and the semi-ellipse shaped melt pool are mainly found on the YZ plane. In this case, the cells will more easily grow along ± 45° from the building direction on the YZ plane under the
combined effects of epitaxial growth and fully remelting of the previously deposited layers, and then leading to a strong cube texture. Fig. 4 presents the EDS element mapping of the selected regions of the SS-X and SS-XY specimens. It reveals that the second phase located in the interdendritic region is enriched in Nb, Mo and depleted in Ni, Cr, and Fe. The chemical composition matches well with that of Laves phase, which was also determined by selected area electron diffraction (SAED) pattern in our previous work [16]. Laves phase is a brittle interdendritic phase that would preferentially form in Inconel 718 due to the micro-segregation [18], while γ′′/γ′ strengthening phases do not appear in the as-built specimens. 3.2. Mechanical properties Fig. 5(a) and (b) present tensile engineering stress-strain curves and a comparison of the tensile strength between the SS-X and SS-XY 44
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Fig. 4. EDS element mapping of the selected regions in the (a) SS-X and (b) SS –XY specimens.
specimens at room temperature, respectively. It is clear that the yield strength and ultimate tensile strength of the SS-X specimens are 10.5% and 13.9% higher than that of the SS-XY specimens (Fig. 5(b)), while the ductility of the former is slightly lower than that of the latter. Fig. 6 shows the relationship between the applied stress amplitude (σa) and the number of cycles to failure (Nf, S-N curves) of the SS-X and SS-XY specimens. The SS-X specimens have longer fatigue lifetime than that of the SS-XY specimens, and also the fatigue limit of the SS-X specimens (252 MPa) is 8.6% higher than that of the SS-XY specimens (232 MPa). 3.3. Fracture behavior Fig. 7 presents morphologies of the side surface and the fracture surface of the SS-X and the SS-XY specimens after tensile test. Obvious necking shown in Fig. 7(a) and (b) demonstrates that these two types of specimens are ductile. Both fine and coarse slip bands were observed on the side surface close to fracture of the specimens, as shown in Fig. 7(c) and (d). Close observations shown that cracks initiated from the Laves phases, which were impinged by the extensive slip bands (Fig. 7(e) and (f)). Fig. 7(g) and (h) show a large number of dimples, indicating a transgranular failure mode. Fig. 8 presents morphologies of the side surface and the fracture surface of the SS-X and SS-XY specimens after fatigue failure. A large number of slip bands were observed and interacted with the Laves phases in the interdendritic regions of the SS-X and SS-XY specimens, as shown in Fig. 8(a) and (b). Due to the inhomogeneous deformation between the matrix and Laves phases, Laves phases were fragmented and easily became the preferential sites for the crack nucleation (Fig. 8(c) and (d)). Fig. 8(e) and (f) show that fatigue cracks of both types of specimens all initiated from the process-induced voids. 4. Discussion
Fig. 5. (a) Tensile engineering stress-strain curves of both types of specimens, (b) comparison of the yield strength and ultimate tensile strength of the SS-X and SS-XY specimens.
The above results clearly indicate that mechanical properties of the SS-X specimens are superior to that of the SS-XY specimens. The possible reasons for the scanning strategy-dependent tensile properties and
fatigue strength will be discussed as follows, including the void size and distribution, dendrite structure and phase composition, crystalline orientation, as well as grain structure. 45
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4.3. Grain orientation To compare the extent of activated slip systems between the SS-X and SS-XY specimens, Taylor factor mappings of the SS-X and SS-XY specimens were constructed based on the loading direction (BD) relative to the {111} < 110 > slip systems of each grain, as shown in Fig. 9(a) and (b), respectively. Furthermore, the distribution of Taylor factor of the SSX and SS-XY specimens is shown in Fig. 9(c) and (d), respectively. The average Taylor factor of the SS-X specimen is 2.95, which is lower than that (3.39) of the SS-XY specimen. It indicates that the SS-XY specimen has higher resistance to deformation than the SS-X specimen when loading along BD. However, the yield strength of the SS-XY specimen (562.22 MPa) is lower than that (621.46 MPa) of the SS-X specimen. This suggests that the higher yield strength of the SS-X specimen does not result from the crystallographic orientation. Such scanning strategy-dependent mechanical properties should be attributed to other factors. 4.4. Grain structure
Fig. 6. Stress amplitude-number of cycles to failure (S-N) curves of the SS-X and SS-XY specimens.
As shown in Fig. 2(a) and (b), the grain structure in the SS-X and SS-XY specimens is obviously different [14]. For the SS-X specimens, small irregular grains were prevalently formed, which is attributed to the potential heterogeneous nucleation caused by the common effects of the complex heat flux direction coupled with the preferred growth direction [20,21]. However, for the SS-XY specimens, due to the pronounced epitaxial growth by fully remelting of the previous deposited layers, the large directional columnar grain structure was developed [14]. The characterization of the YZ cross-section shows that the mean grain size of the SS-XY specimen is almost 2.3 times larger than that of the SS-X specimen, thus the yield strength of the SS-X specimen is 1.5 times higher than that of the SS-XY specimen based on the Hall-Petch relation. Thus, the higher yield strength of the SS-X specimen mainly results from the refinement of the grain size. Actually, the yield strength of the SS-X specimen (621.46 MPa) is only 1.1 times higher than that of the SS-XY specimen (562.22 MPa). It may be attributed to the lower Taylor factor and non-uniform distribution of the grain size in the SS-X specimen. In addition, the uneven deformation may occur among the non-uniformly distributed grains, leading to the degraded ductility of the SS-X specimen [8]. To describe the stress-controlled fatigue life, the Basquin equation was used to fit the S-N curves:
4.1. Void size and distribution SLM-fabricated materials always inevitably contain a large number of process-induced micron-scale voids, which are detrimental to mechanical properties, especially fatigue properties [4,7]. Compared with the failure induced by static loading, fatigue failure is mostly a local phenomenon controlled by defects and microstructure heterogeneities [4,19]. Therefore, the size and distribution of voids should be the primary factor for the variation in fatigue properties of these two types of specimens. However, there is no significant difference in the size and distribution of the voids of the SS-X and SS-XY specimens, as shown in Fig. 1. This indicates that the void size and distribution are not the main reasons responsible for the scanning strategy-dependent mechanical properties. 4.2. Phase composition and dendrite structure For the as-built specimens, Laves phase was found as the only secondary phase, while other phases, such as γ′′/γ′ strengthening phases and δ phases could hardly be observed (Fig. 4). Therefore, the strengthening mechanism for the as-built specimens is mainly dependent on the width of dendrites and grain size rather than the γ′′/γ′ strengthening phases. The mean width of dendrites of the SS-X and SS-XY specimens is 518 ± 143 nm and 528 ± 124 nm, respectively. Therefore, the variation in the mechanical properties of these two types of specimens is not attributed to the phase composition and dendrite structure.
a
=
f
(2Nf ) b,
(1)
where Nf is the number of cycles to failure, σ′f is the fatigue strength coefficient and b is the fatigue strength exponent, respectively. According to the formula, σ′f and b are related to the intercept and the slope of the fitted line in Fig. 6. It can be found that σ′f of the SS-X
Fig. 7. Obvious necking behavior of the (a) SS-X and (b) SS-XY specimens after tensile test, magnified images of the slip bands in the (c) SS-X and (d) SS-XY specimens, cracks initiated from the fragmented Laves phase in the interdendritic region of the (e) SS-X and (f) SS-XY specimens, a large number of dimples of (d) SS X- and (h) SS XY specimens shown on the fracture surface. 46
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Fig. 8. The side surface of the (a) SS-X and (b) SS-XY specimens after fatigue failure, magnified images of the fragmented Laves phases in the interdendritic regions of the (c) SS-X and (d) SS-XY specimens, the fracture surface of the (e) SS-X and (f) SS-XY specimens showing fatigue cracks all initiated from the process-induced voids.
Fig. 9. Taylor factor mappings of the (a) SS-X and (b) SS-XY specimens when loading along BD, the distribution of the Taylor factor of the (c) SS-X and (d) SS-XY specimens.
specimen is higher, but b is slight lower than that of SS-XY specimen. For most metals, σ′f has an intimate relation with the static strength of material and b reflects the damage mechanism related to cyclic deformation [22]. In the case of high-cycle fatigue (HCF), the total fatigue
life is dominated by crack initiation. Process-induced void has a degraded effect on the fatigue crack initiation. However, we assume that the void-degraded extent of fatigue strength of the SS-X and the SS-XY specimens should be same due to the void size and distribution of these 47
Materials Science & Engineering A 753 (2019) 42–48
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two types of specimens are at the same level. The slight decline in b of SS-X specimen compared with that of the SS-XY specimen is attributed to the increase in the level of strain localization with the decrease in grain size in the crack initiation stage [22]. In addition, the fatigue ratio (fatigue strength/ultimate tensile strength) of the SS-X and SS-XY specimens is 0.31 and 0.32, respectively. The identical fatigue ratio further demonstrates that the fatigue strength is closely related to the ultimate tensile strength for the SLM-fabricated Inconel 718 in the as-built state. Thus, the main reason for the higher fatigue strength of the SS-X specimen than that of the SS-XY specimen is mainly attributed to the finer grain size. Based on the above analysis, we can conclude that the grain size has an evident effect on the tensile and fatigue strength of the SLM-fabricated Inconel 718 at room temperature in comparison with the crystalline orientation. This finding may provide the possibility to modify mechanical properties of AM components through tailoring the grain structure with different scanning strategies. Previous investigations have shown that the variation in grain size with different scanning strategies is mainly attributed to the following factors: (a) misorientation angle between the growth direction and the maximum temperature gradient and (b) cooling rate. A large misorientation angle and a high cooling rate were suggested to cause a larger amount of undercooling and increased the driving force for heterogeneous nucleation, which ultimately resulted in fine grain size [23]. However, compared with the tensile and fatigue strength at room temperature, the creep resistance would become degraded due to fine grain structures at high temperature [24,25]. Therefore, it is still necessary to have a comprehensive understanding on the relationship of process (scanning strategies), microstructure (including voids) and mechanical properties (at room and high temperature), which would be helpful to realize site-specific control of the grain structure and mechanical properties.
doi.org/10.1016/j.msea.2019.03.007. References [1] T.S. Srivatsan, T.S. Sudarshan, Additive Manufacturing: Innovations, Advances and Applications, CRC Press, Boca Raton, 2016. [2] X.Q. Wang, X.B. Gong, K. Chou, Review on powder-bed laser additive manufacturing of Inconel 718 parts, Proc. I. Mech. Eng. B-j. Eng. (2015) 1–9. [3] E. Brandl, U. Heckenberger, V. Holzinger, et al., Additive manufactured AlSi10Mg samples using Selective Laser Melting (SLM): microstructure, high cycle fatigue, and fracture behavior, Mater. Des. 34 (2012) 159–169. [4] A. Yadollahi, N. Shamsaei, Additive manufacturing of fatigue resistant materials: challenges and opportunities, Int. J. Fatigue 98 (2017) 14–31. [5] B. Zhang, H. Liao, C. Coddet, Effects of processing parameters on properties of selective laser melting Mg-9%Al powder mixture, Mater. Des. 34 (2012) 753–758. [6] J.A. Cherry, H.M. Davies, S. Mehmood, et al., Investigation into the effect of process parameters on microstructural and physical properties of 316L stainless steel parts by selective laser melting, Int. J. Adv. Manuf. Technol. 76 (2015) 869–879. [7] A. Yadollahi, N. Shamsaei, S.M. Thompson, et al., Effects of building orientation and heat treatment on fatigue behavior of selective laser melted 17-4 PH stainless steel, Int. J. Fatigue 94 (2017) 218–235. [8] F. Liu, X. Lin, C. Huang, et al., The effect of laser scanning path on microstructures and mechanical properties of laser solid formed nickel-base superalloy Inconel 718, J. Alloy. Comp. 509 (2011) 4505–4509. [9] L.N. Carter, C. Martin, P.J. Withers, et al., The influence of the laser scan strategy on grain structure and cracking behaviour in SLM powder-bed fabricated nickel superalloy, J. Alloy. Comp. 615 (2014) 338–347. [10] R. Rashid, S.H. Masood, D. Ruan, et al., Effect of scan strategy on density and metallurgical properties of 17-4PH parts printed by Selective Laser Melting (SLM), J. Mater. Process. Technol. 249 (2017) 502–511. [11] L. Parry, I.A. Ashcroft, R.D. Wildman, Understanding the effect of laser scan strategy on residual stress in selective laser melting through thermo-mechanical simulation, Addit. Manuf. 12 (2016) 1–15. [12] A. Kudzal, B. McWilliams, C. Hofmeister, et al., Effect of scan pattern on the microstructure and mechanical properties of Powder Bed Fusion additive manufactured 17-4 stainless steel, Mater. Des. 133 (2017) 205–215. [13] T. Ishimoto, K. Hagihara, K. Hisamoto, et al., Crystallographic texture control of beta-type Ti–15Mo–5Zr–3Al alloy by selective laser melting for the development of novel implants with a biocompatible low Young's modulus, Scripta Mater. 132 (2017) 34–38. [14] H.Y. Wan, Z.J. Zhou, C.P. Li, et al., Effect of scanning strategy on grain structure and crystallographic texture of Inconel 718 processed by selective laser melting, J. Mater. Sci. Technol. 34 (2018) 1799–1804. [15] L. Thijs, M.L.M. Sistiaga, R. Wauthle, et al., Strong morphological and crystallographic texture and resulting yield strength anisotropy in selective laser melted tantalum, Acta Mater. 61 (2013) 4657–4668. [16] H.Y. Wan, Z.J. Zhou, C.P. Li, et al., Enhancing fatigue strength of selective laser melting-fabricated Inconel 718 by tailoring heat treatment route, Adv. Eng. Mater. (2018) 1800307. [17] T. DebRoy, H.L. Wei, J.S. Zuback, et al., Additive manufacturing of metallic components – process, structure and properties, Prog. Mater. Sci. 92 (2018) 112–224. [18] D. Zhang, W. Niu, X. Cao, et al., Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy, Mater. Sci. Eng. 644 (2015) 32–40. [19] S. Suresh, Fatigue of Materials, second ed., Cambridge University Press, New York, 1998. [20] C. Qiu, C. Panwisawas, M. Ward, et al., On the role of melt flow into the surface structure and porosity development during selective laser melting, Acta Mater. 96 (2015) 72–79. [21] R.W. Messler, Principles of Welding: Processes, Physics, Chemistry, and Metallurgy, Wiley-VCH, Weinheim, 1999. [22] R.H. Li, Z.J. Zhang, P. Zhang, et al., Improved fatigue properties of ultrafine-grained copper under cyclic torsion loading, Acta Mater. 61 (2013) 5857–5868. [23] N. Raghavan, S. Simunovic, R. Dehoff, et al., Localized melt-scan strategy for site specific control of grain size and primary dendrite arm spacing in electron beam additive manufacturing, Acta Mater. 140 (2017) 375–387. [24] E.G. Richards, Influence of specimen size and grain size on creep-rupture strength of some nickel-base high-temperature alloys, J. Inst. Met. 96 (1968) 365–370. [25] Y.S. Lee, D.W. Kim, D.Y. Lee, et al., Effect of grain size on creep properties of type 316LN stainless steel, Met. Mater. Int. 7 (2001) 107–114.
5. Conclusions (1) Scanning strategy plays a significant role in influencing the microstructure and resulting mechanical properties of SLM-fabricated Inconel 718 at room temperature. The tensile strength and fatigue strength of the SS-X specimens are superior to that of the SS-XY specimens. (2) The comprehensive analysis reveals that both the crystalline orientation and grain structure have an effect on mechanical properties of the SLM-fabricated Inconel 718. Compared with the crystalline orientation, the refinement of grain size is responsible for the higher tensile strength and fatigue strength of the SS-X specimens at room temperature than that of the SS-XY ones. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 51771207 and 51571199). We are grateful to Dr. Shaogang Wang for his help on 3D XRT experiments and analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://
48
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of scanning strategy on mechanical properties of selective laser melted Inconel 718
T
H.Y. Wana,b, Z.J. Zhouc, C.P. Lic, G.F. Chenc, G.P. Zhanga,∗ a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016, China c Materials & Manufacturing Qualification Group, Corporate Technology, Siemens Ltd., China, Beijing, 100102, China b
ARTICLE INFO
ABSTRACT
Keywords: Selective laser melting Inconel 718 Scanning strategy Grain size Mechanical properties
The effect of scanning strategy, i.e. bidirectional scanning without (SS-X) and with a 90°-rotation (SS-XY) for every layer, on mechanical properties of Inconel 718 fabricated by selective laser melting (SLM) was investigated. The results show that tensile strength and fatigue strength of SS-X specimens are superior to that of the SS-XY ones. Such excellent mechanical properties of the SS-X specimens at room temperature were found to mainly result from the processing-induced fine grain structures compared with void size, crystalline orientation or dendrite structure.
1. Introduction
2. Experimental
Selective laser melting (SLM), is one of the promising additive manufacturing (AM) technologies, which is capable of fabricating nearnet shape metallic components with geometrically complex structures [1,2]. Despite many advantages, SLM process still faces a lot of challenges that need to be addressed prior to widespread industrial application. Two main challenges are microstructure heterogeneities and randomly dispersed defects, which result in the uncertainty and degradation in mechanical properties [3,4]. To obtain the final components with high density and optimal mechanical properties, much effort has been devoted to optimize the processing parameters, such as laser energy density [5,6], building orientation [7], and scanning strategy [8,9]. Scanning strategy is an important variable that significantly affects the thermal history during the SLM process and further influences the specimen density [10], residual stress [11] and evolution of the microstructures [8], ultimately changes the mechanical properties [12]. Most previous researchers focused on controlling the solidification microstructure and crystalline orientation by tailoring the scanning strategy [9,13–15], while few studies are available on the effect of the scanning strategy on mechanical properties, especially fatigue properties [8,9,12]. In this paper, two types of scanning strategies of SLM were adopted to produce the Inconel 718 specimens. The effects of void size distribution, phase composition and dendrite structure, crystalline orientation, as well as grain structure induced by scanning strategy on tensile and fatigue properties were comprehensively discussed.
2.1. Specimen preparation
∗
Specimens with the dimension of 10 × 10 × 10 mm3 were fabricated by an SLM apparatus (EOSINT M280 400W, EOS, Germany) equipped with an Yb fiber laser source under an argon atmosphere with an oxygen level of 0.1%. As-received Inconel 718 spherical metal powder with a size ranging from 15 μm to 53 μm was produced by Ar gas-atomization and the chemical composition was presented in our previous work [16]. The SLM process parameters, such as laser power, scanning speed, hatch distance and layer thickness were shown in Table 1. These process parameters were optimized according to the following criteria. Firstly, no obvious cracking and delamination can be found. Secondly, no obvious unmelt powder can be found on the surface of build parts. Thirdly, the profile depth, Pt (the sum of the largest profile peak height and the largest profile valley depth within the evaluation length) used to evaluate the surface roughness should be lower than 200 μm. Finally, the relative density should be larger than 99.5%. In order to investigate the effect of scanning strategy on mechanical properties of SLM-fabricated Inconel 718, two types of scanning strategies, bidirectional scanning without and with a 90° rotation between the successive layers, were adopted, which were named as SS-X and SS-XY, respectively. 2.2. Material characterization The size and distribution of voids were detected by the 3D-high
Corresponding author. E-mail address: [email protected] (G.P. Zhang).
https://doi.org/10.1016/j.msea.2019.03.007 Received 11 June 2018; Received in revised form 16 December 2018; Accepted 1 March 2019 Available online 07 March 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 753 (2019) 42–48
H.Y. Wan, et al.
Table 1 Process parameters of the SLM-fabricated Inconel 718. Sample
Laser power (W)
Scanning speed (mm/s)
Hatch distance (mm)
Layer thickness (mm)
Platform temperature (°C)
E = P/vht (J/mm3)
Scanning strategy
SS X SS XY
285 285
960 960
0.1 0.1
0.04 0.04
80 80
74.2 74.2
S-parallel (0°) S-cross (90°)
resolution transmission X-ray tomography (XRT) technique. The pixel size of 2.1 μm was selected depending on the specimen volume. Electron backscatter diffraction (EBSD) was performed to analyze the grain structure and crystalline orientation, the threshold value for the grain size was a misorientation of 10°. Dendrite structures were characterized by a field-emission scanning electron microscope (FE-SEM, LEO Supra 35) and the second phases were determined by a fieldemission transmission electron microscope (FE-TEM, FEI Tecnai 20) with an energy dispersive X-ray spectroscopy (EDS) attachment. 2.3. Tensile and fatigue test Dog-bone shaped tensile specimens with gauge dimensions of 3 mm in length, 1 mm in width and 200 μm in thickness were prepared. Tensile tests were conducted at a strain rate of 5 × 10−4 s−1 along building direction (BD) of the SLM-fabricated Inconel 718 using an INSTRON 5848 universal test machine with a 2 kN load cell at room temperature. The tensile strain was measured by a non-contact laser extensometer (MTS LX 300). Fatigue test specimens have a hourglassshaped test section with dimensions of 1 mm in width and 200 μm in thickness. Fatigue tests were also performed along BD at room temperature in an INSTRON E 1000. A stress ratio and loading frequency is 0.1 and 30 Hz, respectively. Both tensile and fatigue specimens were extracted from the inside of build cubes and mechanically polished followed by electropolished for 1 min at 12 V in a 10% perchloric acid solution to avoid the effect of surface defects. The side surface and fracture surface of failure specimens were characterized by the FE-SEM. 3. Results 3.1. Void distribution and microstructure Fig. 1(a) and (b) show 3D-XRT images on the SS-X and SS-XY specimens, respectively, indicating that a large number of micron-sized spherical voids uniformly distributed in both types of specimens. The relative density of the SS-X and SS-XY specimens is calculated as 99.85% and 99.81%, respectively. Fig. 1(c) shows that the size and distribution of voids have no remarkable difference. The mean void size of the SS-X and the SS-XY specimens is 9.3 ± 5.6 μm and 9.1 ± 5.5 μm, respectively. Fig. 2(a) and (b) present the grain structure and crystallographic texture of the SLM-fabricated Inconel 718 from the YZ cross section, respectively. The grain structure of the SS-X and SS-XY specimens is distinctive and the mean intercept length perpendicular to the BD is calculated as 19.06 μm and 43.90 μm, respectively (see Fig. 2(a) and (b)). While the mean width of dendrite structure shown in Fig. 2(c) and (d) is almost same (∼500 nm). As shown in the insets of Fig. 2(a) and (b), crystallographic textures of the SS-X and SS-XY specimens were evidently different. A weak texture was developed when scanning strategy X was applied. It is because that the elongated direction of the cells largely deviated from the plane perpendicular to the scanning direction (X) due to the complex heat flux direction, which is caused by the combined effects of Marangoni flow and vapor recoil pressure [14]. On the contrary, scanning strategy XY gave rise to a strong cube texture, which is different from that by Ishimoto et al. in which strong < 001 > alignments along the X, Y and Z directions [13]. It is also unexpected to find that the macroscopic morphology on the XZ plane and YZ plane in scanning strategy XY are significantly different, as shown in Fig. 3(a) and (b). The alternative morphology of the laser tracks and semi-ellipse
Fig. 1. 3D-XRT images showing the distribution of the voids of the (a) SS-X and (b) SS-XY specimens, (c) void size distribution of both types of specimens.
shaped melt pools can hardly be found. Instead, the laser tracks can only be found on the XZ plane (Fig. 3(a)) and the semi-ellipse shaped melt pools can only be observed on the YZ plane (Fig. 3(b)). However, the semi-ellipse shaped melt pools are visible on the top deposited layers on the XZ plane (Fig. 3(c) and (d)), which is similar to that found on the YZ plane (Fig. 3(b)). Therefore, the differences (macroscopic morphology and texture) between the XZ plane and YZ plane in the scanning strategy XY in our present work are attributed to other factors. 43
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Fig. 2. Band contrast (BC) maps showing grain structure of the (a) SS-X and (b) SS-XY specimens from the YZ cross section, insets in (a) and (b): pole figures of the SSX and SS-XY specimens, respectively, high-magnification SEM backscatter electron images of the dendrite structure of the (c) SS-X and (d) SS-XY specimens. Fig. 3. Cross-sectional pictures taken in the (a) XZ plane and (b) YZ plane of the SS-XY specimen shown the laser track morphology and semi-ellipse shaped melt pool morphology, respectively, (c) image of the cross-sections of the top layers taken in the XZ plane of the SS-XY specimen, (d) magnified images of the area enclosed in red rectangle in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
It should be noted that, besides the scanning strategy, the amount of partial remelting of the previously deposited layers and/or neighboring tracks (depending on the material system and specific processing conditions, such as scanning speed and layer thickness) would influence on the formation of macroscopic morphology and texture [17]. If the next deposited layer does not fully remelt the previously deposited layer, the alternative morphology of laser tracks and semi-ellipse melt pools can be observed on both XZ and YZ planes. And the mechanism for the texture formation in the scanning strategy XY will follow Ishimoto's opinion. By contrast, if the next deposited layer almost fully remelts the previously deposited layer, the macroscopic morphology on the XZ and YZ planes will be different. In other words, the laser tracks are mainly observed on the XZ plane and the semi-ellipse shaped melt pool are mainly found on the YZ plane. In this case, the cells will more easily grow along ± 45° from the building direction on the YZ plane under the
combined effects of epitaxial growth and fully remelting of the previously deposited layers, and then leading to a strong cube texture. Fig. 4 presents the EDS element mapping of the selected regions of the SS-X and SS-XY specimens. It reveals that the second phase located in the interdendritic region is enriched in Nb, Mo and depleted in Ni, Cr, and Fe. The chemical composition matches well with that of Laves phase, which was also determined by selected area electron diffraction (SAED) pattern in our previous work [16]. Laves phase is a brittle interdendritic phase that would preferentially form in Inconel 718 due to the micro-segregation [18], while γ′′/γ′ strengthening phases do not appear in the as-built specimens. 3.2. Mechanical properties Fig. 5(a) and (b) present tensile engineering stress-strain curves and a comparison of the tensile strength between the SS-X and SS-XY 44
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Fig. 4. EDS element mapping of the selected regions in the (a) SS-X and (b) SS –XY specimens.
specimens at room temperature, respectively. It is clear that the yield strength and ultimate tensile strength of the SS-X specimens are 10.5% and 13.9% higher than that of the SS-XY specimens (Fig. 5(b)), while the ductility of the former is slightly lower than that of the latter. Fig. 6 shows the relationship between the applied stress amplitude (σa) and the number of cycles to failure (Nf, S-N curves) of the SS-X and SS-XY specimens. The SS-X specimens have longer fatigue lifetime than that of the SS-XY specimens, and also the fatigue limit of the SS-X specimens (252 MPa) is 8.6% higher than that of the SS-XY specimens (232 MPa). 3.3. Fracture behavior Fig. 7 presents morphologies of the side surface and the fracture surface of the SS-X and the SS-XY specimens after tensile test. Obvious necking shown in Fig. 7(a) and (b) demonstrates that these two types of specimens are ductile. Both fine and coarse slip bands were observed on the side surface close to fracture of the specimens, as shown in Fig. 7(c) and (d). Close observations shown that cracks initiated from the Laves phases, which were impinged by the extensive slip bands (Fig. 7(e) and (f)). Fig. 7(g) and (h) show a large number of dimples, indicating a transgranular failure mode. Fig. 8 presents morphologies of the side surface and the fracture surface of the SS-X and SS-XY specimens after fatigue failure. A large number of slip bands were observed and interacted with the Laves phases in the interdendritic regions of the SS-X and SS-XY specimens, as shown in Fig. 8(a) and (b). Due to the inhomogeneous deformation between the matrix and Laves phases, Laves phases were fragmented and easily became the preferential sites for the crack nucleation (Fig. 8(c) and (d)). Fig. 8(e) and (f) show that fatigue cracks of both types of specimens all initiated from the process-induced voids. 4. Discussion
Fig. 5. (a) Tensile engineering stress-strain curves of both types of specimens, (b) comparison of the yield strength and ultimate tensile strength of the SS-X and SS-XY specimens.
The above results clearly indicate that mechanical properties of the SS-X specimens are superior to that of the SS-XY specimens. The possible reasons for the scanning strategy-dependent tensile properties and
fatigue strength will be discussed as follows, including the void size and distribution, dendrite structure and phase composition, crystalline orientation, as well as grain structure. 45
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4.3. Grain orientation To compare the extent of activated slip systems between the SS-X and SS-XY specimens, Taylor factor mappings of the SS-X and SS-XY specimens were constructed based on the loading direction (BD) relative to the {111} < 110 > slip systems of each grain, as shown in Fig. 9(a) and (b), respectively. Furthermore, the distribution of Taylor factor of the SSX and SS-XY specimens is shown in Fig. 9(c) and (d), respectively. The average Taylor factor of the SS-X specimen is 2.95, which is lower than that (3.39) of the SS-XY specimen. It indicates that the SS-XY specimen has higher resistance to deformation than the SS-X specimen when loading along BD. However, the yield strength of the SS-XY specimen (562.22 MPa) is lower than that (621.46 MPa) of the SS-X specimen. This suggests that the higher yield strength of the SS-X specimen does not result from the crystallographic orientation. Such scanning strategy-dependent mechanical properties should be attributed to other factors. 4.4. Grain structure
Fig. 6. Stress amplitude-number of cycles to failure (S-N) curves of the SS-X and SS-XY specimens.
As shown in Fig. 2(a) and (b), the grain structure in the SS-X and SS-XY specimens is obviously different [14]. For the SS-X specimens, small irregular grains were prevalently formed, which is attributed to the potential heterogeneous nucleation caused by the common effects of the complex heat flux direction coupled with the preferred growth direction [20,21]. However, for the SS-XY specimens, due to the pronounced epitaxial growth by fully remelting of the previous deposited layers, the large directional columnar grain structure was developed [14]. The characterization of the YZ cross-section shows that the mean grain size of the SS-XY specimen is almost 2.3 times larger than that of the SS-X specimen, thus the yield strength of the SS-X specimen is 1.5 times higher than that of the SS-XY specimen based on the Hall-Petch relation. Thus, the higher yield strength of the SS-X specimen mainly results from the refinement of the grain size. Actually, the yield strength of the SS-X specimen (621.46 MPa) is only 1.1 times higher than that of the SS-XY specimen (562.22 MPa). It may be attributed to the lower Taylor factor and non-uniform distribution of the grain size in the SS-X specimen. In addition, the uneven deformation may occur among the non-uniformly distributed grains, leading to the degraded ductility of the SS-X specimen [8]. To describe the stress-controlled fatigue life, the Basquin equation was used to fit the S-N curves:
4.1. Void size and distribution SLM-fabricated materials always inevitably contain a large number of process-induced micron-scale voids, which are detrimental to mechanical properties, especially fatigue properties [4,7]. Compared with the failure induced by static loading, fatigue failure is mostly a local phenomenon controlled by defects and microstructure heterogeneities [4,19]. Therefore, the size and distribution of voids should be the primary factor for the variation in fatigue properties of these two types of specimens. However, there is no significant difference in the size and distribution of the voids of the SS-X and SS-XY specimens, as shown in Fig. 1. This indicates that the void size and distribution are not the main reasons responsible for the scanning strategy-dependent mechanical properties. 4.2. Phase composition and dendrite structure For the as-built specimens, Laves phase was found as the only secondary phase, while other phases, such as γ′′/γ′ strengthening phases and δ phases could hardly be observed (Fig. 4). Therefore, the strengthening mechanism for the as-built specimens is mainly dependent on the width of dendrites and grain size rather than the γ′′/γ′ strengthening phases. The mean width of dendrites of the SS-X and SS-XY specimens is 518 ± 143 nm and 528 ± 124 nm, respectively. Therefore, the variation in the mechanical properties of these two types of specimens is not attributed to the phase composition and dendrite structure.
a
=
f
(2Nf ) b,
(1)
where Nf is the number of cycles to failure, σ′f is the fatigue strength coefficient and b is the fatigue strength exponent, respectively. According to the formula, σ′f and b are related to the intercept and the slope of the fitted line in Fig. 6. It can be found that σ′f of the SS-X
Fig. 7. Obvious necking behavior of the (a) SS-X and (b) SS-XY specimens after tensile test, magnified images of the slip bands in the (c) SS-X and (d) SS-XY specimens, cracks initiated from the fragmented Laves phase in the interdendritic region of the (e) SS-X and (f) SS-XY specimens, a large number of dimples of (d) SS X- and (h) SS XY specimens shown on the fracture surface. 46
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Fig. 8. The side surface of the (a) SS-X and (b) SS-XY specimens after fatigue failure, magnified images of the fragmented Laves phases in the interdendritic regions of the (c) SS-X and (d) SS-XY specimens, the fracture surface of the (e) SS-X and (f) SS-XY specimens showing fatigue cracks all initiated from the process-induced voids.
Fig. 9. Taylor factor mappings of the (a) SS-X and (b) SS-XY specimens when loading along BD, the distribution of the Taylor factor of the (c) SS-X and (d) SS-XY specimens.
specimen is higher, but b is slight lower than that of SS-XY specimen. For most metals, σ′f has an intimate relation with the static strength of material and b reflects the damage mechanism related to cyclic deformation [22]. In the case of high-cycle fatigue (HCF), the total fatigue
life is dominated by crack initiation. Process-induced void has a degraded effect on the fatigue crack initiation. However, we assume that the void-degraded extent of fatigue strength of the SS-X and the SS-XY specimens should be same due to the void size and distribution of these 47
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two types of specimens are at the same level. The slight decline in b of SS-X specimen compared with that of the SS-XY specimen is attributed to the increase in the level of strain localization with the decrease in grain size in the crack initiation stage [22]. In addition, the fatigue ratio (fatigue strength/ultimate tensile strength) of the SS-X and SS-XY specimens is 0.31 and 0.32, respectively. The identical fatigue ratio further demonstrates that the fatigue strength is closely related to the ultimate tensile strength for the SLM-fabricated Inconel 718 in the as-built state. Thus, the main reason for the higher fatigue strength of the SS-X specimen than that of the SS-XY specimen is mainly attributed to the finer grain size. Based on the above analysis, we can conclude that the grain size has an evident effect on the tensile and fatigue strength of the SLM-fabricated Inconel 718 at room temperature in comparison with the crystalline orientation. This finding may provide the possibility to modify mechanical properties of AM components through tailoring the grain structure with different scanning strategies. Previous investigations have shown that the variation in grain size with different scanning strategies is mainly attributed to the following factors: (a) misorientation angle between the growth direction and the maximum temperature gradient and (b) cooling rate. A large misorientation angle and a high cooling rate were suggested to cause a larger amount of undercooling and increased the driving force for heterogeneous nucleation, which ultimately resulted in fine grain size [23]. However, compared with the tensile and fatigue strength at room temperature, the creep resistance would become degraded due to fine grain structures at high temperature [24,25]. Therefore, it is still necessary to have a comprehensive understanding on the relationship of process (scanning strategies), microstructure (including voids) and mechanical properties (at room and high temperature), which would be helpful to realize site-specific control of the grain structure and mechanical properties.
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5. Conclusions (1) Scanning strategy plays a significant role in influencing the microstructure and resulting mechanical properties of SLM-fabricated Inconel 718 at room temperature. The tensile strength and fatigue strength of the SS-X specimens are superior to that of the SS-XY specimens. (2) The comprehensive analysis reveals that both the crystalline orientation and grain structure have an effect on mechanical properties of the SLM-fabricated Inconel 718. Compared with the crystalline orientation, the refinement of grain size is responsible for the higher tensile strength and fatigue strength of the SS-X specimens at room temperature than that of the SS-XY ones. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 51771207 and 51571199). We are grateful to Dr. Shaogang Wang for his help on 3D XRT experiments and analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://
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